In semiconductor manufacturing process, native oxide typically forms when a semiconductor substrate surface is exposed to a surrounding containing oxygen and/or moisture. Oxygen exposure occurs when substrates are moved between processing chambers at atmospheric or ambient conditions, or when a small amount of oxygen remains in a processing chamber.
Native oxide films are usually very thin, for example between 5-20 angstroms, but thick enough to cause difficulties in subsequent fabrication processes. Furthermore, native oxide may cause high contact resistance in source and drain areas and adversely increase the thickness of equivalent of oxide (EOT) in channel areas. Therefore, a native oxide layer is typically undesirable and needs to be removed prior to subsequent fabrication processes.